The present invention relates to photolithography.
Photolithography is widely used to form patterns on semiconductor wafers during fabrication of integrated circuits. A wafer 110 (FIG. 1) is coated with a photoresist layer 120. Resist 120 is irradiated from a light source 130. A mask or reticle 140 is placed between source 130 and resist 120. Mask 140 carries a pattern consisting of opaque and clear features. This pattern defines which areas of resist 120 are exposed to the light from source 130. After the exposure, the resist 120 is developed so that some of the resist is removed to uncover the underlying substrate 110. If the resist is xe2x80x9cpositivexe2x80x9d, then the resist is removed where it was exposed to the light. If the resist is xe2x80x9cnegativexe2x80x9d, the resist is removed where it was not exposed. In either case, the remaining resist and the exposed (uncovered) areas of substrate 110 reproduce the pattern on mask 140. The wafer is then processed as desired (e.g. the exposed areas of substrate 110 can be etched, implanted with dopant, etc.).
The resist pattern on wafer 110 is not always a faithful reproduction of the mask. In FIG. 2, an opaque feature 210M on mask 140 is a narrow line having two ends 220M. Line 210M should ideally be printed (reproduced) in resist 120 as line 210R, with ends 220R. In fact, the resist underlying the line 210M near the ends 210R gets overexposed. As a result, the line 210R is shortened and the line ends are smoothened in the resist pattern, as shown at 240.
The resist pattern can be corrected with hammerheads 310 (FIG. 3). The hammerheads are opaque regions appended at line ends 210M to reduce the resist exposure near the line ends. However, if the line ends 220M are close to other opaque features, the hammerheads can be difficult to form on the mask.
The invention is defined by the appended claims which are incorporated into this section in their entirety. The rest of this section summarizes some features of the invention.
Some embodiments of the present invention provide alternative techniques to reduce overexposure of the resist. In some embodiments, the resist is exposed twice. One exposure is through a mask like in the prior art, for example, in FIG. 2 or 3, but the light dose is smaller. The other exposure is conducted through a different (xe2x80x9cmodifiedxe2x80x9d) mask which covers some or all of the resist at the line ends. The total energy delivered to the resist at the line ends is smaller as a result.
Other features and embodiments are described below.